Nov 28 (Reuters) - As the nations of the world struggle in Doha to agree even modest targets to tackle global warming, the cuts needed in rising greenhouse gas emissions grow ever deeper, more costly and less likely to be achieved.

U.N. talks have delivered only small emissions curbs in 20 years, even as power stations, cars and factories pump out more and more heat-trapping gases.

An overriding long-term goal set by all nations two years ago to keep temperature rises to less than 2 degrees Celsius (3.6 F) above levels prior to the Industrial Revolution is fast slipping away.

"The possibility of keeping warming to below 2 degrees has almost vanished," Pep Canadell, head of the Global Carbon Project at Australia's Commonwealth Scientific and Research Organization, told Reuters.

Disagreements mean the U.N. climate talks in Doha, Qatar, that run until Dec. 7 have scant chance of making meaningful progress. The talks are aimed at reaching a new deal to start by 2020 to slow climate change in the form of more floods, droughts, rising sea levels and severe storms like Hurricane Sandy that lashed the U.S. Northeast last month.

Global emissions of carbon dioxide (CO2), the main greenhouse gas, have risen 50 percent since 1990 and the pace of growth has picked up since 2000, Canadell said. In the past decade, emissions have grown about 3 percent a year despite an economic slowdown, up from 1 percent during the 1990s.

Based on current emissions growth and rapid industrial expansion in developing nations, emissions are expected to keep growing by about 3 percent a year over the next decade.

For the talks to have any chance of success in the long run, emissions must quickly stop rising and then begin to fall. Temperatures have already risen by 0.8 C (1.4 F) since pre-industrial times.

"The alarm bells are going off all over the place. There's a disconnect between the outside world and the lack of urgency in these halls," Alden Meyer of the Union of Concerned Scientists said at the Doha talks.

Nearly 1,200 coal-fired power plants, among the biggest emitters, are proposed around the globe, with three-quarters of them planned for China and India, a study by the Washington-based World Resources Institute think-tank said last week.

Emissions from China, the world's top carbon polluter, are growing 8 to 9 percent a year and are now about 50 percent higher than those of the United States. And China's carbon emissions are not expected to peak until 2030.

POLLUTION

In some projections, global emissions will need to go into reverse by mid-century, with the world sucking more carbon out of the air than it puts in, if warming is to be kept to below 2 C.

And air pollution, mostly particles from fossil fuel use, may be masking the warming by dimming sunshine.

Without that pollution, a breach of the 2 degree threshold might already be inevitable, he said.

The latest IPCC report, in 2007, said keeping greenhouse gas concentrations low would cost less than 3 percent of world gross domestic product by 2030. So far, the panel has not assessed the costs of delays, said Rajendra Pachauri, chairman of the panel.

The report also said that world emissions of greenhouse gases would need to peak by 2015 to give a good chance of keeping the average temperature rise to below 2 C.

But deep disagreement on future emissions cuts between rich and poor nations has delayed the start of a new global pact until 2020, undermining the chances of a robust extension in Doha of the existing plan, the Kyoto Protocol, which obliges almost 40 rich nations to cut emissions until the end of 2012.

The deadline for a deal on new cuts due to start in 2020 has been put back to 2015, giving breathing space for the troubled talks as ever more carbon enters the air.

Yet current emissions cut pledges are putting the planet on course for a warming of 3 to 5 C, a U.N. report said last week, adding that 2 C was still possible with tough action.

"The later we go in getting complete action and the higher emissions are in 2020, the greater is the risk that these targets are not possible or are extremely expensive," said Bill Hare, head of the non-profit advisory organisation Climate Analytics.

Key will be a switch to nuclear or biomass power and carbon capture and storage. If these don't step up, there will be no financially feasible solutions to meet the target, he said.

In Doha, both the United States and the European Union - the main emitters among developed nations - say they will not deepen their pledges for cuts by 2020. "It's a desperate situation," said Martin Kaiser of Greenpeace.

To be effective, the next climate pact from 2020 would need global agreement for rapid and deep cuts. Under a scenario drawn up by the IPCC, rich nations needed to achieve cuts of 25 to 40 percent by 2020 from 1990 levels.

But existing pledges are for less than 20 percent.

STARK MESSAGE

Canadell, citing work by the Global Carbon Project and other researchers, said that to have a reasonable chance of keeping warming to 2 C, global emissions would have to drop about 3 percent a year from 2020.

Since developed nations are meant to take the lead, that would mean the rich would have to cut by between 4 and 5 percent a year, he said. That could cripple economies by prematurely shutting down coal-fired power plants and polluting factories.

Global accountancy firm PwC estimated that the improvement in global carbon intensity - the amount of carbon emitted per unit of economic output - needed to meet a 2 C target had risen to 5.1 percent a year, from now to 2050.

"We have passed a critical threshold - not once since World War Two has the world achieved that rate of decarbonisation, but the task now confronting us is to achieve it for 39 consecutive years," PwC said.

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